1. Field of the Invention
The present invention generally relates to a variable gain amplifier and, more particularly, to a variable gain amplifier for adjusting a level of an analog image-signal component contained in a video signal supplied by, for example, a CCD linear image sensor so as to convert the image signal component which corresponds to an amount of an input light into digital data. Such a variable gain amplifier is used in a CCD analog-signal processing circuit which supplies an analog signal to an A/D converter converting an image signal into digital image data.
2. Description of the Related Art
FIG. 1 shows a first example of a conventional variable gain amplifier that comprises an amplifier 2 and an R-2R ladder 11 serving as a signal attenuator. FIG. 2 is a circuit diagram of the R-2R ladder 11. Additionally, FIG. 3 shows a second example of a conventional variable gain amplifier that comprises an operational amplifier 12 and the R-2R ladder 11 as a signal attenuator provided in a feedback loop of the operational amplifier 12.
FIG. 4 shows a third example of a conventional gain amplifier that comprises the amplifier 2 and a dB-attenuator 13. The dB-attenuator 13 may be a .pi.-type or a T-type. FIG. 5 is a circuit diagram of the dB-amplifier 13 of a .pi.-type. As shown in FIG. 5, the dB-amplifier 13 comprises a plurality of .pi.-type amplifiers so that a necessary number of the .pi.-type amplifiers through which a signal is passed are selected from among the plurality of .pi.-type amplifiers. FIG. 6 shows a fourth example of a conventional variable gain amplifier that comprises the operational amplifier 12 and the dB-attenuator 13 provided in a feedback loop of the operational amplifier 12. Additionally, there is an amplifier as a fifth example of a conventional variable gain amplifier that comprises a voltage controlled variable gain amplifier and a D/A converter.
According to the above-mentioned first and second examples of the conventional variable gain amplifier, a variable range of a gain is very large and a number of variation steps is small. However, according to the second example, since a gain setting curve is inverse proportional to a setting data curve, a high-speed gain setting can be achieved by rendering an image signal after A/D conversion, that is, a peak value of image data to be the setting data.
According to the above-mentioned third and fourth examples of the conventional variable gain amplifier, a variable range of a gain and a number of variation steps can be freely set. Accordingly, a fine gain setting can be done with a minimum setting-data length, that is, a minimum number of bits. However, if the number of variation steps is increased and a width of each step is set to a small value, a resistance range of a resistor included in the attenuator 13 must be increased. That is, a high resistance is required for the resistor included in the attenuator 13. This causes a problem when the variable gain amplifier is formed in a monolithic IC.
According to the fifth example of the conventional variable gain amplifier, a variable range of a gain, a number of variation steps and a setting curve can be freely set. However, it is difficult to achieve high accuracy fundamentally. If an R-2R ladder having a high accuracy is connected to an amplifier or an operational amplifier, an accurate IC process is required. Additionally, in this case, an ON resistance of each analog switch for selecting resistors included in the R-2R ladder must be decreased, that is, the size of the element constituting each analog switch must be increased. Accordingly, it is difficult to achieve a variable gain amplifier having a high accuracy, or such an amplifier must be very expensive if it can be made.